In charged-particle-beam (CPB) projection microlithography as used in the fabrication of integrated circuits, a circuit pattern defined by a reticle or mask is irradiated with a charged-particle beam to transfer the pattern defined by the reticle or mask to a sensitized substrate (e.g., a semiconductor wafer). In recent years, CPB projection-microlithography apparatus ("pattern-transfer apparatus") have been developed that exhibit improved resolution of the transferred pattern and improved product throughput (i.e., the number of semiconductor wafers that can be exposed with a pattern per unit time). With certain conventional CPB pattern-transfer apparatus, one or more entire die patterns defined on a mask are transferred onto the wafer in a single exposure. A "die" is a pattern coextensive with the bounds of an integrated circuit or other device to be transferred onto the wafer (usually multiple dies are exposed at respective locations on the wafer).
It is difficult to produce a mask for a CPB pattern-transfer apparatus that transfers multiple dies or even an entire die in a single exposure while also providing the high resolution and circuit densities demanded in recent years. In addition, conventional CPB pattern-transfer apparatus that transfer multiple dies or an entire die pattern per exposure cannot satisfactorily control aberrations that arise in the CPB optical system through which the charged-particle beam passes, especially over a large optical field covering one or more dies. To solve this problem, CPB pattern-transfer apparatus have been proposed in which a pattern to be transferred is divided into multiple field segments ("mask subfields") that are exposed individually and separately. Such a pattern is transferred typically using a "step-and-repeat" transfer scheme in which the individual mask subfields are sequentially transferred to corresponding "substrate subfields" on a wafer or other sensitized substrate. The substrate subfields are produced on the wafer surface in locations relative to each other such that the substrate subfields are "stitched" together in the correct order and alignment to reproduce the entire die pattern on the wafer surface (e.g., see U.S. Pat. No. 5,260,151).
A demagnifying (or "reduction") pattern-transfer apparatus irradiates a charged-particle beam onto a portion of a mask defining a circuit pattern of an entire die. An image of the die pattern is then demagnified and formed on the wafer (e.g., see Japanese Laid Open Patent Document No. HEI 5-160012). Because a die pattern image cannot be transferred to the wafer with sufficiently high resolution when irradiating the entire die pattern in a single exposure, a step-and-repeat transfer scheme is used to transfer the die pattern to the wafer subfield-by-subfield.
A mask for use with a "partial-batch" pattern-transfer method defines a repeating pattern portion of an integrated circuit, such as a DRAM, and non-repeating patterns. The mask is irradiated and the pattern portion to be repeatedly transferred to the wafer is reduced and transferred thereto the desired number of times. The non-repeating pattern portions of the mask undergo direct writing to the wafer (i.e., the patterns are not reduced), typically using a conventional variable-shaped-beam method. The partial-batch pattern transfer method improves wafer throughput in comparison to a variable-shaped-beam method.
In conventional CPB projection-microlithography apparatus employing a reduction pattern-transfer method, the transferred image typically encounters linear distortion. For example, linear distortion of a square image may result in a transferred image having a rectangular or parallelogram transverse profile rather than a square transverse profile. Conventionally, astigmatic-aberration correction coils are used in CPB projection apparatus to correct linear distortion of the projected images. The correction coils normally are situated in any of various locations in such apparatus. (The optimal positions of the correction coils is unclear from the teachings of the prior art). With the prior art apparatus, correction of linear distortion using astigmatic-aberration correction coils results in an image experiencing secondary astigmatic aberration (astigmatic blur), producing a transferred image having poor resolution due to blurring of the image edges. However, attempts to correct the astigmatic blur of the image edges typically result in further linear distortion of the image.
Further, the conventional method used to measure linear distortion of a projected image involves projection of an enlarged image onto a fluorescent-light panel. The projected image is then viewed by eye to determine the extent the image has experienced linear distortion (e.g., J. Vac. Sci. Tech. 16 (60):1723 November/December 1979). Accordingly, to perform a conventional linear-distortion measurement method, two lenses and a fluorescent-light panel must be placed on a lower portion of a sensitive-substrate mounting surface. Such apparatus requirements result in a larger, more complex, and expensive projection apparatus. Additionally, because the extent of linear distortion of a projected image is determined by eye, the prior art methods for measurement of linear distortion lack precision and/or accuracy.
Accordingly, there is a need for CPB projection-microlithography apparatus and transfer methods for transferring a pattern from a mask onto a sensitized substrate with precise correction of linear distortion of the transferred images and without the creation of significant astigmatic blur of the transferred image.